In conventional bag making wherein a tube is formed from sheet material having a heat sealable material on a face thereof, the tube is continuously moved in an axial direction. A fin-type seal can be formed wherein the inner heat sealable face of one edge portion of the sheet overlaps the inner heat sealable face of the opposite edge portion which is folded back over the body of the tube. A heat sealing die pressing against the fin, heat seals such edges together against a back-up plate heretofore located within the tube.
The heat sealable material is on the inside face of the tube which is the face that necessarily slides over the back-up plate when located within the tube. Heat from a heat sealing die penetrates the fin with the result that the inner heat sealable face of the tube becomes softened and scuffed as it rides over the back-up plate. This frequently results in seal failure and mistracking of the fin over the back-up plate.